SATA II is going to have 300MB/s max transfer rate. You have to look at your drive’s maximum throughput and read/write throughput, and not just max external transfer rate which is only part of the picture. I looked for a listing of various 7200rpm drives - the one I found is a little dated, but you were seeing max in the 120MB/s range, which is not going to be impacted by the SATA II transfer rate. There are faster HDDs now, but as far as I know, none of them are above 300MB/s so I wouldn’t worry about it. However, as you noted, SATA II could be a bottleneck for a SSD, so if there is a chance that you will want to upgrade to a SSD, you should go SATA III.